Electronically tuned ultra high frequency television tuner

ABSTRACT

An ultra high frequency (UHF) tuner includes tunable transmission lines having two conductive sections disposed on one face of a dielectric plate opposite a conductive ground plane disposed on the other dielectric plate face. The transmission lines are tuned by variable capacitance devices coupled between the conductive section of each line. To minimize interaction between the two transmission lines, the conductive sections of the lines are disposed on opposite surfaces of the dielectric plate with like ends of the lines adjacent opposite edges of the plate.

United States Patent Carlson Mar. 14, 1972 [54] ELECTRONICALLY TUNEDULTRA FOREIGN PATENTS OR APPLICATIONS HIGH FREQUENCY TELEVISION1,100,751 1/1968 Great Britain ..334/15 TUNER [72] Inventor: David JohnCarlson, Indianapolis, Ind. Pri y mmin Herman Karl Saalbach r AssistantExaminer-Paul L. Gensler [73] Assignee. RCA Corporation Aumey Eugene M-whimcre [22] Filed: Mar. 23, 1970 21 Appl. No.: 21,563 [57] ABSTRACT Anultra high frequency (UHF) tuner includes tunable trans- 52 US. Cl...334/15, 325/445, 331/1 17 D, mission lines having two conductiveSections disposed on one 333 2 3, 333 34 M, 334 41 face of a dielectricplate opposite a conductive ground plane [51] Int. Cl. ..H03h 5/12disposed on the other dielectric plate face. The transmission [58] Fieldof Search ..333/738, 82 B, 82 A, 84 M; lines are tuned by variablecapacitance devices coupled 331/117 325/422, 445 between the conductivesection of each line. To minimize interaction between the twotransmission lines, the conductive [56] References cued sections of thelines are disposed on opposite surfaces of the UNITED STATES PATENTSdielectric plate with like ends of the lines adjacent opposite edges ofthe plate. 2,545,623 3/1951 Mackenzie ..333/82 A X Dachert ..307/320 X32 (Ilaims, 22 Drawing Figures l A i 9/- I i I l I PATENTEBHARMIQYESHEET 1 OF 6 N. JM 8 s:

kkww SQ DAVID JOHN CAR LSON AP as Ne .wllilllllllll PATENTEDHAR 14 1972SHEET 2 OF 6 FigZ.

/74 I54 154, m9 l INVENTOR DAVID J HN CARLSON HQMZW ATTORNEY PATENTEUMAR14 I972 SHEET '4 [IF 6 INVEN'IOR DAVID JOHN CARLSON ATT ORN EIYELECTRONICALLY TUNED ULTRA HIGH FREQUENCY TELEVISION TUNER The presentinvention relates to ultra high frequency (UHF) tuners, and moreparticularly to UHF tuners which are tuned by voltage responsivevariable capacitance devices.

UHF tuners used in television receivers are generally of the typeincluding a conductive housing having a number of components enclosingcapacity end-loaded unbalanced transmission line structures, Typically,each transmission line includes a conductor suspended within thecompartment and endloaded by a parallel plate air dielectric capacitor.The capacitor includes stator plates which are attached to one end ofthe transmission line conductor and rotor plates. The rotor plates ofthe capacitors for each transmission line are mounted on a tuning shaftwhich passes through the several tuner housing compartments.

The UHF television band encompasses 70 channels making it difficult toprovide a reliable inexpensive preset tuning mechanism which provides atuning operation similar to the detented step-by-step very highfrequency (VHF) tuning (Channels 2-13) on most television receivers. Inaddition, the manufacture of the present types of UHF tuners requiresconsiderable hand labor and close tolerances must be maintained becauseof the frequency ranges involved.

A UHF tuner embodying the invention is tuned by voltage responsivecapacitance means such as variable capacitance diodes. Since the tuningoperation does not require the precision mechanical presetting ofavariable capacitor shaft, preset tuning operations may be provided moreconveniently.

In addition, a UHF tuner embodying the invention can be made by massproduction techniques, and does not require as much precision hand laboras is required in the manufacture of tuners of the type now available.The tuner includes transmission lines formed on a dielectric plate. Eachtransmission line includes a first and a second conductive sectiondisposed on one side of the plate opposite a ground plane disposed onthe other side of the dielectric plate. A tuning capacitance device,such as a variable capacitance diode, couples the first and secondconductive sections of each of the transmission lines. In accordancewith one aspect ofthe invention, isolation may be provided between twoof the transmission lines by locating the conductive sections for thetransmission lines on opposite sides of the plate. In accordance with afurther feature ofthe invention, corresponding ends of the two lines tobe isolated are directed toward opposite edges ofthe plate.

In accordance with a feature of the present invention, a double tunedcircuit may be formed on the dielectric plate. The double tuned circuitincludes a pair of transmission lines each having a first and a secondconductive section coupled by a variable capacitance device and disposedon one face of the dielectric plate opposite a conductive ground planedisposed on the other dielectric plate face. Uniform coupling betweenthe two circuits which are both tunable across a wide band offrequencies may be obtained by two coupling means. A first couplingmeans interconnects the first conductive sections of each of the twocircuits to provide dominant coupling toward the lower end of thedesired band while the second coupling means interconnects the secondsections of each circuit to provide dominant coupling toward the upperend ofthe band.

A complete understanding of the present invention may be obtained fromthe following detailed description of a specific embodiment thereof,when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram of a UHF television tunerembodying the present invention;

FIG. 2 is a perspective view, partially broken away, of the tunerschematically shown in FIG. 1;

FIG. 3 is a bottom view of the tuner shown in FIG. 2;

FIG. 4 is a left side view with the tuner cover and chassis frame brokenaway to expose the tuner components;

FIG. 5 is a right side view of the tuner shown in FIG. 2 with the tunercover and chassis frame broken away to show the tuner components;

FIG. 6 is a plan view of the tuner substrate and pattern shown in FIG.4, drawn to scale, with all the tuner components and the substratecoating material removed;

FIG 7 is a plan view of the tuner substrate and patterns shown in FIG.5, drawn to scale, with all the tuner components and the substratecoating material removed;

FIG. 8 is a series of curves showing plots of tuning capacity as afunction of resonant frequency for the tunable resonant circuits of thetuner;

FIG. 9 is an enlarged partial section view of the substrate showingdetails of the tuner;

FIGS. 10a-c are enlarged partial section views of the substrate showingone of the adjustable tracking inductors set for minimum, nominal andmaximum inductance;

FIGS. lla-e are a series of curves showing standing voltage waveshelpful in understanding the operation of the tuner;

FIGS. IZa-e are a series of curves showing standing waves ofcurrentcorresponding to the curves shown in FIG. 11.

Referring now to the drawings wherein like reference numerals designatesimilar elements in the various views, a UHF television tuner 50 isenclosed in a metal housing 52 which is maintained at a referencepotential, shown as ground. The UHF tuner includes an RF amplifier stage54, an oscillator stage 56, a mixer stage 58, and an IF amplifier stage60. UHF television signals are intercepted by an antenna, not shown, andapplied to a UHF input terminal 62. The input signals are amplified inthe amplifier stage 54 and heterodyned in the mixer stage 58 withlocally generated signals from the oscillator stage 56 to produce anintermediate frequency signal which is thereafter amplified in the IFamplifier stage 60 to produce an amplified intermediate frequency signaloutput at an IF output terminal 64.

The tuner includes four tunable resonant circuits 66, 68, 70 and 72. Thetunable resonant circuit 66 is associated with the RF amplifier inputcircuitry, while the tunable resonant circuits 68 and 70 are part ofadouble tuned interstage network between the RF amplifier stage 54 andthe mixer stage 58. The tunable resonant circuit 72 is used to establishthe frequency of oscillation of the oscillator stage 56.

The tunable resonant circuits 66, 68, 70 and 72 include transmissionline structures which are tuned by variable capacitance diodes. All ofthe transmission line structures include conductive elements formed onboth faces of a dielectric plate. Tunable resonant circuit 66 includesaligned transmission line sections 67a and 67b; tunable resonant circuit68 includes the transmission line sections 69a and 69b,- tunableresonant circuit 70 includes the transmission line sections 710 and 71b;and finally, tunable resonant circuit 72 includes the transmission linesections 73a and 73b. One end of the second line sections 67b, 69b, 71band 73b is connected to the point of reference potential, Each pair ofline sections cooperate with the ground plane on the opposite side ofthe dielectric plate to operate as transmission lines.

The two sections of each composite transmission line are coupled byvariable capacitance tuning diodes 75, 79, 83 and 87 and adjustabletracking inductors 77, 81, 85 and 89, respectively. Each of the seriesconnected variable capacitance diodes 75, 79, 83 and 87 exhibit acapacitance whose magnitude varies inversely with the magnitude ofreverse bias applied across the variable capacitance diode. The tunableresonant circuits 66, 68 and 70 are apportioned to tune across afrequency band ranging from 470 MHz. through 890 MHz., while the tunableresonant circuit 72 associated with the oscillator stage 56 isapportioned to tune across a band of frequencies ranging from 517 MHz.through 931 MHz.

Each composite transmission line is apportioned so that the secondsections 67b, 69b and 71b of the line are one quarter wavelengthresonant at a frequency above 890 MHz., the highest desired frequency towhich the tunable resonant circuit must tune. The first transmissionline sections 67a, 69a and 71a are apportioned to be half wavelengthresonant above the highest frequency to which the tunable resonantcircuit must tune, i.e., 890 MHz. In a like manner, the second sectionof transmission line 73b associated with the oscillator tunable resonantcircuit 72 is apportioned to be one quarter wavelength resonant at afrequency above 93] MHz., while the first transmission line section 73ais apportioned to be half wavelength resonant above 931 MHz.

The resonant frequency of each section may be measured by electricallydisconnecting the variable capacitance tuning diode and adjustabletracking inductor and thereafter coupling a unit impulse of energy intothe section under investigation. The unit impulse will cause the sectionto ring simultaneously at several related frequencies, which can bemeasured, for example, by a sampling oscilloscope. The fundamentalresonant frequency is the lowest frequency present in the ringingsection. The mode of resonance can be determined by measuring thestanding wave ratios along the section to determine the voltage maximaand null points.

A dielectric plate or substrate 91, which supports the compositetransmission lines, is mounted in a conductive enclosure (FIG. 2). Theenclosure includes detachable covers 99 and 101 and a chassis or framemember 97. Two ground plane sections 93 and 95 are disposed on oppositesides of the substrate 91. The composite transmission lines 69, 71 and73 include and are disposed opposite the ground plane section 95, whilethe RF input composite transmission line 67 includes and is disposedopposite the ground plane section 93. The substrate 91 and itsconductive areas are shown in FIGS. 6 and 7, which are drawnapproximately to scale. The substrate height is 3,375 inches and thesubstrate width is 3.500 inches. While the several RF compositetransmission lines 67, 69, and 71 are designed to resonate atapproximately the same frequency for a given diode capacitance, theydiffer slightly in size to compensate for the effects introduced by thedifferent tuner components connected as shown in FIGS. 4 and 5.

The substrate 91, which is about 50-milli-inches thick, is fabricatedfrom an aluminum oxide consisting of approximately 85% AL O and 15percent mixture of calcium oxide, magnesium oxide and silicon dioxide. Aconductive pattern, about 0.0005-inches thick, is disposed on both thesubstrate faces and consists of silver and glass which has been fused at900 C. The entire pattern is covered by a copper plating 0.0002 to0.0005 inches thick. A moisture and solder resistant silicon, modifiedto harden, is applied to the entire substrate and copper plated pattern,with the exception of bonding pads used to electrically connect thetuner components to the substrate pattern. One suitable modified siliconis manufactured by Electroscience Corporation and designated 240-SB. Theexposed bonding pads on the substrate facilitates rapid and accurateassembly of the tuner. in FIGS. 2, 4 and 5, the conductive sections onthe substrate (the transmission line sections, the ground planesections, and the capacitor plates associated with the oscillatorcircuit) are shown crosshatched to indicate the, insulative coatingwhich normally covers these components has been removed.

Shaping of each composite transmission line section 67b, 69b and 71bprovides a relative tracking between the tunable resonant circuits 66,68 and 70 and oscillator tunable resonant circuit 72. The shaping is inthe form of an exponential taper between the grounded and diode ends ofeach section. Because of the exponential tapers, the impedance versusfrequency characteristic of each of the composite transmission lines 67,69 and 71 is modified. Consequently, the effects of a given capacitancechange on tuning frequency varies across the frequency band resulting insimilar curvatures for the plots of tuning capacity as a function ofresonant frequency for the RF tunable resonant circuits 66, 68 and 70and the oscillator tunable resonant circuit 72. The similar curvaturesare shown in FlG. 8 wherein curve (a) represents the plot of tuningcapacity as a function of resonant frequency for the oscillator tunableresonant circuit 72 and curves (b,) (0,) and (d) represent the plot oftuning capacity as a function of resonant frequency for the RF tunableresonant circuit 66 for different inductance settings of the adjustabletracking inductor 77, minimum, nominal and maximum. The adjustabletracking inductors will be discussed in greater detail hereinafter.Since the curvatures of the plots for the two tunable resonant circuitsare similar, tracking of the resonant circuits is provided across theentire desired frequency band of each circuit.

The resonant frequency of each of the transmission lines is determinedby its total reactance which includes the reactive impedances of theupper and lower aligned sections, the variable capacitance diode and theadjustable tracking inductor. The reactive contribution of the uppersection varies in a nonlinear manner with frequency, while the reactivecontribution of the variable capacitance diode and adjustable trackinginductor provides capacitive reactance whose magnitude is determined bythe tuning voltage (identical variable capacitive diodes having the sametuning voltage impressed across them may be used in all the tunableresonant circuits). By adjustment of the tuning voltage the capacitivereactance is varied and tunes the transmission line across the band offrequencies. For proper tracking between the oscillator and RF tunableresonant circuits, the oscillator tunable resonant circuit must resonateabove the RF tunable resonant circuits by a given constant amount forany given tuning voltage adjustment. The dissimilarly shaped lowersections of the RF signal selection and oscillator tunable resonantcircuits cause the rate of change of the total reactance with frequencyto be modified. Specifically, the lower section of each RF transmissionline includes an exponential taper and the lower section of theoscillation transmission line includes a substantially linear taper.Consequently, these sections differ in rate of reactance change withfrequency from each other and from their respective upper sections. Thiscauses the total reactance of each transmission line to vary withfrequency in a manner which provides tracking between the RF andoscillator tunable resonant circuits. It should be noted that theseveral tapered edges on the upper section of each of the transmissionlines compensate for the effects of fringing of the electromagnetic andelectrostatic fields at the section ends.

While shaping of the composite transmission line sections 67b, 69b and71b provides a first order relative tracking of each of the several RFtunable resonant circuits with the oscillator tunable resonant circuit,nevertheless, the tunable resonant circuits must still be aligned withrespect to each other to compensate for part tolerances. That is, theplots representing the capacitive characteristic of each resonantcircuit must be properly centered, frequency wise, with respect to theother tunable resonant circuits.

lt has been determined that the series inductance of the lead wires ofeach of the variable capacitance diodes 75, 79, 83 and 87 is asignificant parameter in determining the resonant frequency for a givendiode capacitance, particularly at the lower end of the UHF frequencyhand. For example, an increase in variable capacitance diode 75 leadlengths of less than 0.1 inch results in a several picofarad reductionin capacitance required by the tunable resonant circuit 66 for it toresonate at 470 MHz. This series inductive effect provides a potentialsource of detuning between the several tunable resonant circuits 66, 68,and 72 as well as variation from one tuner to the next. The inductiveeffect, however, may be controiled and utilized to provide a means forcentering or aligning the tunable resonant circuits.

An aperture is provided in the substrate 91 for each of the variablecapacitance diodes 75, 79, 83 and 87. Referring to FIG. 9 which is anenlarged partial section view of the substrate 91 showing a portion ofthe composite transmission line 67, variable capacitance diode ispositioned in an aperture 750 in the substrate 91. The hole 750 providesa location means for the body of the variable capacitance diode 75 andpermits accurate positioning of the components.

The diode 75 is secured to two bondings pads 75b and 75c on oppositesides of the aperture 75a. The bonding pad 75c is an area on the secondsection of transmission line while the bonding pad 75!) is a separateconductive pad. The bondings pads 75b and 750 are spaced a predetermineddistance apart and help minimize the series inductance variations byproviding a control for the lead lengths of the variable capacitancediode 75. Moreover, the aperture 75a in the substrate material 91reduces the dielectric adjacent the body of the diode 75 to therebyminimize the distributed shunt capacitance between the ends of the diodeand also eliminates the need to bend the diode leads (increasing itsinductance) during mounting of the components.

The adjustable tracking inductor 77 is connected in series between thebonding pad 751; and one end of the first section of the compositetransmission line 67a. The inductor 77 consists of a thin wide strip ofcopper which may be adjusted to change its inductance. To changeinductance, the configuration of the loop may be changed from a tallthin structure for minimum inductance to a more circular structure formaximum inductance. This is most clearly shown in FIGS. l0a-c where theadjustable tracking inductor 77 is shown set for minimum, nominal andmaximum inductance, respectively. The series adjustable inductor foreach of the composite transmission lines 67, 69, 71 and 73 swamps minorinductance variations due to the diode lead length and provides acontrollable series inductive effect.

Centering of the tracking for each of the tunable resonant circuits 66,68, 70 and 72 is obtained by adjusting the shape of the inductive loopassociated with each composite transmission line. The effect ofadjusting the inductor 77 is shown in FIG. 8 where the three plots oftuning capacity as a function of resonant frequency (b, c, and d)represent the effects of setting the adjustable tracking inductor 77between its minimum, nominal and maximum inductance positions,respectively. The inductive loops are adjusted such that a properconstant frequency separation is obtained between the resonantfrequencies of the RF tunable resonant circuits and the oscillatortunable resonant circuit across their frequency bands.

Received UHF television signals applied at the input terminal 62 arecoupled through a high pass filter comprising the inductors 74 and 76and the capacitor 78, to the RF amplifier input circuit 66. The highpass filter functions to pass frequencies within the UHF frequency band;that is, frequencies ranging from 470 MHz. to 890 MHz. The tunableresonant circuit 66 is coupled via a capacitor 80 to the emitterelectrode of a grounded base transistor amplifier 82. The transistor 82is shown encapsulated in a conductive housing which is connected toground by lead 102 to reduce the likelihood of parasitic oscillations.

Operating potential for the transistor 82 is obtained from a source ofB+ applied to a terminal 84 which is bypassed to ground for radiofrequencies by a feedthrough capacitor 103. The potential is applied tothe collector electrode of the transistor 82 through a radio frequencydecoupling inductor 86, a resistor 88, and an RF choke 90. The choke 90is a single component including a kilo-ohm resistor providing the wirewinding form for an inductor, both of which are electrically connectedin parallel. The resistor reduces the figure of merit or Q of the choketo reduce the possibility of spurious parasitic resonances. The emitterelectrode of the transistor 82 is connected to ground by a resistor 92to complete the collector-emitter DC current path.

Bias to the base electrode of the transistor 82 is provided from thesource of operating potential applied at the terminal 84 through thecollector-emitter current path of an automatic gain control transistor94. An automatic gain controlling potential is applied to the baseelectrode of the transistor 94 via a terminal 96. Terminal 96 isbypassed to ground for radio frequency signals by a feedthroughcapacitor 105. The automatic gain control transistor 94 controls thebase bias to the RF amplifier transistor 82, and thus, the RF amplifierstage gain. Transistor 94 is connected as an emitter-follower so thatsubstantial isolation is provided between the automatic gain controlcircuits and the RF amplifier 82. Further RF isolation for the 13+supply and the AGC circuitry is provided by two feedthrough capacitors98 and 100, respectively. The feedthrough capacitor 100 additionallyprovides a low impedance RF path to ground for the base electrode oftransistor 82 establishing the grounded base mode of operation.

A capacitor 104 couples the collector electrode of the RF amplifiertransistor 82 and the tunable resonant circuit 68. Signals developed inthe tunable resonant circuit 68 are inductively coupled to the tunableresonant circuit 70 by the inductors 106 and 108. The inductor 106provides the dominant coupling toward the lower end of the UHF frequencyband, while the inductor 108 provides the dominant coupling toward thehigher end of the UHF frequency band. The tunable resonant circuits 68and 70 with the coupling inductors 106 and 108 combine to form a doubletuned interstage network interconnecting the RF amplifier stage 54 andthe mixer stage 58.

The mixer stage 58 includes a mixer diode 110 having its cathodeconnected to a tap point 112 in the tunable resonant circuit 70. Theanode of the mixer diode 110 is connected by a pickup loop 114, aninductor 116 and a capacitor 118 to the input of the IF amplifier stage60, terminal 119-119. inductor 116 and capacitor 118 are apportioned totransform the diode output impedance to match the IF amplifier stageinput impedance. A DC bias is applied to the mixer diode 110 from the B+supply to maintain a DC current flow of approximately 1.5 milliamperesthrough the mixer diode. The bias to the diode is applied from theterminal 84 through the inductor 86 and to series connected resistors120-122, and the pickup loop 114 to the anode of the mixer diode 110.The cathode of the diode is returned to ground through a portion of thetunable resonant circuit 70.

Amplified UHF signals are applied to the mixer diode 110 from thetunable resonant circuit 70 at the tap connection 112. An oscillatorwave is applied to the mixer diode from the oscillator stage 56 so thatthe mixer diode heterodynes the amplified UHF signals and the locallygenerated signal to provide a desired IF output signal. The oscillatorsignal is coupled from the tunable resonant circuit 72 to the pickuploop 114 connected to the anode of the mixer diode 110. A feedthroughcapacitor 124 coupled between the inductive pickup loop 114 and thepoint of reference potential is selected to provide a low impedance pathto ground for both the amplified UHF signals and the oscillator signaland a higher impedance path for IF signals, As a result, intermediatefrequency signals generated in the mixer diode 110 are passed andapplied to the IF amplifier stage 60 for amplification.

The oscillator stage 56 includes a transistor 126 connected as amodified colpitts oscillator whose frequency is determined by thetunable resonant circuit 72. Operating potential for the oscillatortransistor 126 is provided by the B+ supply via the terminal 84, theinductor 86 and the resistor 120 to a junction 128 which is bypassed toground for UHF waves by a feedthrough capacitor 130. The potential atthe junction 128 is applied to the collector electrode of the oscillatortransistor 126 through a resistor 132 and an RF choke 134. A DC emitterground return for the transistor is provided by a resistor 136. Basebias is obtained through the voltage divider resistors 138 and 140,connected between the junction 128 and ground. A capacitor 142 connectsthe base electrode of the transistor 126 and ground to provide afrequency dependent signal path between the base electrode and ground.

A capacitor 144 couples the collector electrode of transistor 126 to thetunable resonant circuit 72. To sustain oscillation, a portion of thevoltage developed at the collector electrode of the transistor iscoupled to the emitter electrode of the transistor through a capacitivevoltage divider including the three capacitors 146, 148 and 150. Topermit a wide range of Gm transistors to be utilized in the oscillatorstage, capacitor 148 is selected to roll off the high frequency responseof the transistor. Consequently, the capacitor 148 is selected to belossy; that is, have a frequency dependent resistive component causingresistive loading of the oscillator transistor at the higherfrequencies. One suitable capacitor is an 0.82 pf, type GA, capacitormanufactured by the Stackpole Corporation.

Since tunable resonant circuit 72 includes a low impedance, aluminadielectric, transmission line, a relatively large value couplingcapacitor 144 (as compared to the typical UHF television tuner highimpedance air dielectric, half wave transmission line) is required forimpedance matching purposes. This necessitates large capacitors in thecapacitive voltage divider to provide the proper signal feedbackvoltages.

The capacitors 144, 146 and 150 are conductive areas formed on thesubstrate 91 (FIGS. 4 and 5). The capacitor 144 consists ofa conductivearea 501 formed over a conductive area 503 on the opposite side of thesubstrate within a window 505 in the ground plane 95. Capacitor 146consists of a conductive area 503 cooperating with a conductive area 507disposed within the window 505 adjacent area 503, and capacitor 150consists of a conductive area 507 cooperating with the adjacent portionof the ground plane 95 to the right of the conductive area as viewed inFIG. 5. The capacitors 144, 146 and 150 may be fabricated, as otherconductive areas, by printed circuit techniques. This assures that eachof the several capacitances is accurately and consistently reproduced inmass production. As a result of the capacitance uniformity from tuner totuner, the possibility of inoperative or degraded tuners due tocomponent variations or misalignment during assembly is substantiallyreduced.

The oscillator tunable resonant circuit 72 exhibits an undesiredresonance at about 1,400 MHz. The parasitic resonant frequency is notsubstantially affected by the capacitance of the variable capacitancediode 87. With the component values shown, it has been found that theundesired resonant frequency changes by approximately 60 MHz. with acapacitive varia tion of approximately 13 pf.

It will be noted that the parasitic resonant frequency of theoscillators composite transmission line is a second harmonic frequencycentered on approximately 700 MHz. which is within the desired UHFoscillator frequency band. A reduction of fundamental frequencyoscillator signal voltage is observed as the oscillator tunable resonantcircuit 72 is adjusted to resonate within this vicinity. This reducesthe available oscillator signal which may be coupled to the tuner mixerdiode 110. It is believed that the reduction of the fundamentalfrequency oscillator signal voltage is due to a suck out effect causedby the parasitic circuit.

To prevent parasitic resonance and minimize the voltage reduction, thefirst section 730 of the oscillators composite transmission line iscoupled to the oscillator transistor 126 at the parasitic frequencyvoltage null point. This results in minimum spurious signal energytransfer from the tunable resonant circuit 72 through the couplingcapacitor 144 to the oscillator transistor 126.

As the ground plane section 95 associated with the oscillator compositetransmission line is not infinite in size and conductivity, currentflows in the ground plane establishing voltages. A potential couplingpath is provided for coupling these voltages from the ground planesection 95 through, capacitor 142 to the base electrode of theoscillator transistor. Where the current flow in the ground plane is dueto the parasitic resonance, the coupling path tends to encourage theparasitic mode of resonance. This occurs because the spurious signalwhich is applied to the transistor base electrode establishes abase-collector electrode differential voltage which is introduced intothe oscillator feedback network. To minimize this effect, the capacitor142 is positioned on the ground plane section 95 directly over theparasitic null point on the first section of the oscillator compositetransmission line.

The capacitor 142 consists of a bare disc 509 (FIG. 5). The disc 509 isof dielectric material having conductive areas disposed on oppositefaces. The base electrode of transistor 126 is electrically connected toone of the conductive faces while the opposite conductive face ispositioned on the ground plane section over the null point. Bypositioning the capacitor 142 in this manner, a minimum voltage gradientof spurious signal is applied across the transistor collector-basejunction via the two capacitors 142 and 144 which connect theseelectrodes to the resonant circuit. As a consequence, the spuriousvoltage which is introduced in the feedback path is minimized.

As is most clearly shown in FIGS. 4 and 5, no shield walls are providedbetween the tunable resonant circuits of the UHF tuner 50. That is, theRF tunable resonant circuit 66, the interstage tunable resonant circuits68 and 70, and the oscillator tunable resonant circuit 72 are notcompartmentalized in conductive enclosures to prevent interactionbetween the several resonant circuits, and more importantly, to preventa radiation of oscillator energy through the RF tunable resonant circuit66 and out the UHF antenna. However, the tuner 50 is provided with apartial inner oscillator conductive cover 550 (FIG. 2) which overliesthe oscillator transmission line sections 73a-73b. The inner partialcover 550, because it is permanently secured as part of the tunerchassis frame 97, minimizes possible detuning effects of distancevariations between the oscillator stage 56 and detachable tuner covers99 and 101 after removal and reattachment.

The high permeability of the alumina substrate in conjunction with theclose spacing between the composite transmission lines and theirassociated ground plane sections confines the electromagnetic fields.Nevertheless, a fringing of the electromagnetic fields, althoughsubstantially diminished, still occurs. The fringing effect of thefields can cause the oscillator energy to be coupled to the RF tunableresonant circuit 66 to be radiated via the UHF antenna. Moreover, thecoupling can adversely affect the automatic gain control characteristicsof the tuner.

The undesired effects of oscillator radiation are eliminated bydisposing the composite transmission line of the RF tunable resonantcircuit 66 on the opposite side of the alumina substrate 91 from thedouble tuned interstage and oscillator composite transmission lines 69,71 and 73. The ground plane sections 93 and 95 are, likewise, disposedon opposite sides of the alumina substrate. In this manner, theeffectiveness of the electromagnetic and electrostatic coupling betweenthe tunable resonant circuit 66 and the remaining tunable resonantcircuits ofthe tuner 50 is minimized.

Further significant isolation between the RF tunable resonant circuit 66and the remaining tunable resonant circuits of the tuner 50 is achievedby inverting the RF composite transmission line with respect to theinterstage and oscillator composite transmission lines. Thus, the secondshaped section 67b of the RF composite transmission line is disposedtoward the top of the substrate while the first section 67a of the RFcomposite transmission line is disposed toward the bottom of thesubstrate. In contrast, the oscillator and interstage compositetransmission lines each have their second section disposed toward thebottom of the alumina substrate with their first section disposed towardthe top.

For impedance matching purposes, the emitter electrode of the RFtransistor 82 is coupled to the low impedance shaped section 67b of theRF input composite transmission line 67 and the collector electrode oftransistor 82 is coupled to the high impedance section 690 of theinterstage composite trans mission line 69. By having the compositetransmission lines 67 and 69 disposed in inverted relationship, aspreviously described, it is possible to utilize very short lengths forthe RF transistor 82 emitter and collector electrode coupling leads.

The IF amplifier stage 60 includes a transistor 152 mounted external tothe conductive housing 52 and connected as a grounded base amplifier.External mounting of the transistor tends to prevent an undesiredinteraction between the IF amplifier stage and the RF amplifier andmixer stages. The IF input signals are applied to the transistor emitterelectrode, and the collector electrode is connected to the IF outputterminal 64 by a double tuned IF bandpass filter. A feedthroughcapacitor 154 provides a radio frequency bypass to ground for thetransistors base electrode. To minimize the effects of high frequencyparasitic oscillatory circuit paths, a ferrite bead 155 is applied tothe collector electrode of the transistor 82.

The first section of the double tuned IF bandpass filter includes afeedthrough capacitor 156, an inductor 158 and a feedthrough capacitor160. The second section of the double tuned bandpass filter includes thefeedthrough capacitor 160, an inductor 162 and the capacitors 164 and166; capacitor 160, common to both filters, provides the requisitesignal coupling between the sections of the filter. A standoff terminal163 provides a small capacitance mechanical support for the junction ofthe inductor 162 and capacitor 164. Resistive loading of the filters(resistors 172, 174 and an IF signal cable, not shown, coupled toterminal 64) is selected so that the signal response of the IF amplifierstage 60 is flat across the entire desired IF band. That is, equalamplification of signal voltages is provided between both ends of theintermediate frequency band (approximately 41 MHz. to 46 MHz.). Theshaped IF response commonly associated with television intermediatefrequency amplifiers is achieved in later IF stages associated with thetelevision receiver chassis and the VHF tuner. In the latter case, theVHF tuner may be used to provide additional amplification of the UHFtuner IF output signal.

The IF bandpass filter transforms the output impedance of the groundedbase IF amplifier transistor 152 to a resistive output of 75 ohms at thecenter frequency of the IF band, 43 MHZ. This is achieved by adjustingthe tuning slugs in inductors 158 and 162 while applying an IF inputsignal at test point terminal 169. Although the impedance transformationprovided by the bandpass filter is frequency dependent, the deviationfrom 43 MHz. to the upper and lower ends of the IF band is notsufficient to materially change the nature of the output impedance atthe terminal 64. Specifically, the impedance at both the high end andthe low end of the IF frequency band remains predominantly a resistiveimpedance of 75 ohms.

When the tuner IF output terminal 64 is coupled to succeeding lFamplifying stage associated with the television receiver chassis by a75-ohm coupling cable, the impedance looking into the terminal 64closely matches the characteristic impedance of the cable and noreflections occur back along the cable. As a result, any lengthofcoupling cable can be used to couple signals between the televisiontuner and chassis. Naturally, termination of the cable on the televisionchassis must, likewise, be a 75-ohm resistive load. Moreover, becauseresistive coupling is provided between the tuner 50 and the televisionchassis, any capacitive variations which occur due to coupling cabledress do not detune the coupling link as there is no inductance withwhich the capacitance can resonate. Consequently, the dress of the IFcoupling cable is not critical to proper performance of the tuner. Itshould be recognized that since an amplified IF output signal isprovided by the tuner 50, any minor losses in the resistive coupling arenot significant.

Operating potential for the IF amplifier transistor 152 is obtained fromthe B+ supply at the terminal 84, through the inductor 86, an RFisolation inductor I68 and the inductor 158 to the collector electrodeof the transistor 152. A resistor 170 is connected between the emitterelectrode of the transistor and ground to complete the DC current path.Base bias for the transistor 152 is provided by a voltage dividerincluding the resistors I72 and 174 connected between the inductor 158and ground.

A source of variable DC tuning voltage 175 for biasing the variablecapacitance diodes associated with the four tunable resonant circuitshas an internal resistance of 1,000 ohms and is connected betweenterminal 176 and ground. The terminal 176 is bypassed for radiofrequency signals by a feedthrough capacitor 177. The tuning voltage isapplied via the resistors I78 and 180 to ajunction 190 which provides acommon point of tuning potential for the four tunable resonant circuits.The junction 190 is coupled to the tunable resonant circuit 66 via theresistors 180 and 179 and to the tunable resonant circuit 70 via theresistor 182. The junction I90 voltage applied to the tunable resonantcircuit 70 is applied to the tunable resonant circuit 68 via theinductor 106. The junction 190 is also coupled to the tunable resonantcircuit 72 by resistor 185, a resistor 187 and the RF choke 188. Threefeedthrough capaciiii) tors 184, I86 and 183 cooperate with theresistors 180 and 185 to prevent RF and oscillator signal energy frombeing coupled via the DC tuning line between the several tunableresonant circuits and into the source of tuning voltage 175.

With the component values shown, a variable capacitance diode having acapacitance range of approximately 13 picofarads will permit the RFtunable resonant circuits 66, 68, and and the oscillator tunableresonant circuit to be tuned across their respective frequency bands.One suitable variable capacitance diode is the BA I41 diode manufacturedby the International Telephone & Telegraph Corporation. The BA 141 diodeprovides a capacitance ranging from 15 picofarads to 2.3 picofarads asthe tuning voltage is adjusted between approximately 1 and 25 volts DC.

The tuning of the tunable resonant circuits (transmission lines) may beunderstood by reference to FIGS. 11 and 12 showing the standing waves ofvoltage and current, respectively, along the RF input compositetransmission line 67 which is shown at the top of the FIGURES. To tunethe transmission line 67 to the highest frequency within the RF UHF band(FIG. 11b), a voltage is applied across the variable capacitance diodesuch that it exhibits a predetermined capacitance. This capacitancecauses the composite transmission line to resonate with a voltage nullon the transmission line section 6711 located at a point between thecenter and the diode end of the section.

An increase in the voltage across the diode 75 reduces the diodecapacitance and causes the composite transmission line 67 to resonate ata higher frequency. The voltage null on the transmission line section67a displaces toward the center of the section (FIG. 11a). A reductionin the voltage across the diode 75 increases the capacitance and causesthe composite transmission line 67 to resonate at a lower frequency. Thevoltage null on the transmission line section 67a displaces toward thediode end of the section. The amount of frequency change for a givencapacitance increase is dependent upon the characteristic impedance ofthe transmission line which is a function of the width of the line, thespacing from the ground plane and the dielectric of the interveningmedium.

As the voltage across the diode 75 is further reduced, lowering theresonant frequency of the composite transmission line, a point isreached, approximately near in the middle of the desired frequency band(FIG. where the diode capacitance series resonates with the inductanceof the adjustable tracking inductor 77 and the transmission line section67b. At this time, the voltage null on the transmission line section 67ais completely displaced to the diode end of the section.

A still further reduction of the voltage across the diode 75 continuesto lower the resonant frequency of the composite transmission line 67(FIGS. 11b and e). The voltage at the diode end of the transmission linesection 67a increases and the composite transmission line 67 resonatesin a modified one-quarter wavelength mode.

The positioning of the variable capacitance diode 75 away from thegrounded end of the composite transmission line 67 helps maintain a highfigure of merit. This is because the variable capacitance diode 75 islocated at a lower current point as compared to the grounded end of thecomposite transmission line (FIG. 12). As a result, 1 R diode losses areminimized.

At the low end of the frequency band the oscillator diode 87 has areverse bias of approximately 1.0 volt. The oscillator voltage developedacross the diode is of sufficient amplitude during a portion of eachcycle to exceed the diode reverse bias causing rectification of theoscillator voltage. The rectified voltage increases the reverse biasdecreasing the diode 87 capacitance. This in turn causes the tunableresonant circuit 72 to become tuned to a different frequency. Norectification occurs in the RF tunable resonant circuits 66, 68 and 70because the RF UHF signal in these circuits is in the order ofmillivolts as opposed to the order of approximately 1.0 volt in thetunable resonant circuit of the oscillator. To minimize the detuningeffect, the total resistance to ground from the diode 87 through the DCtuning line and the source of tuning voltage 175 is selected to be smallcompared to the oscillator stage driving resistance. in this manner, thetuning voltage at the terminal 176 predominates in controlling thevoltage across the diode because the diode current flowing through thetotal resistance sets up a relatively small voltage which isinsufficient to appreciably change the average DC voltage across thediode.

What is claimed is:

l. A tunable resonant circuit comprising:

a dielectric plate;

an electrically conductive material disposed on one surface of saidplate to define an area of reference potential;

electrically conductive material disposed on the opposite surface ofsaid plate and overlying said area of reference potential defining firstand second spaced conductive transmission line sections;

a voltage responsive capacitance device coupled between said first andsecond sections and adapted to tune said tunable resonant circuit over adesired band of frequencies;

said second section selected to be one-half wavelength reso nant abovethe highest frequency in said desired band of frequencies, and saidfirst section selected to be onequarter wavelength resonant at afrequency above the highest frequency in said desired band offrequencies;

means connecting said first transmission line section to said area ofreference potential; and

means including said transmission line sections for applying a voltageto said voltage responsive capacitance device to control the resonantfrequency of said tunable resonant circuit.

2. A tunable resonant circuit as defined in claim 1 wherein said pair ofspaced transmission line sections are elongated axially aligned sectionsextending from one edge of said plate toward the opposite edge thereof,said first transmission line section connected to said area of referencepotential by a conductive path around said one edge of said plate.

3. A tunable resonant circuit as defined in claim 1 wherein said secondtransmission line conductive section is dimensioned to be one-halfwavelength resonant above 890 MHz. and said first transmission lineconductive section is selected to be one-quarter wavelength resonantabove 890 MHz.

4. A tunable resonant circuit as defined in claim 1 wherein said secondtransmission line conductive section is dimensioned to be one-halfwavelength resonant above 931 MHz. and said first transmission lineconductive section is selected to be one-quarter wavelength resonantabove 931 MHz.

5. A tuner comprising:

a dielectric plate having a first and a second face;

a first and a second conductive section disposed on said first face ofsaid dielectric plate;

a first conductive ground plane disposed on said second face of saiddielectric plate opposite said first and said second conductivesections; first variable capacitance device coupled between said firstand said second sections such that said first and said second sectionsand said first ground plane cooperate to form a first tunable resonantcircuit;

a third and a fourth conductive section disposed on said second face ofsaid dielectric plate;

a second conductive ground plane disposed on said first face of saiddielectric plate opposite said third and fourth sections; and

a second variable capacitance device coupled between said third andfourth sections such that said third and said fourth sections and saidsecond ground plane cooperate to form a second tunable resonant circuit.

6. A tunable resonant circuit as defined in claim 5 wherein said firstand said second conductive sections are axially aligned and said thirdand said fourth conductive sections are axially aligned.

7. A tunable resonant circuit as defined in claim 6 wherein oneconductive section of each tunable resonant circuit resonates with avoltage null on said section above a predetermined frequency and theother conductive section is electrically connected to the oppositelydisposed conductive ground plane.

8. A tunable resonant circuit as defined in claim 7 wherein said voltagenull occurs on said one conductive section at a point located betweenthe center and variable capacitance device end of said section.

9. A tunable resonant circuit as defined in claim 8 wherein saidpredetermined frequency is approximately the center frequency of saiddesired band of frequencies.

10. A tunable resonant circuit as defined in claim 9 wherein said firstconductive section is selected to be one-half wavelength resonant above890 MHz. and said second conductive section is selected to beone-quarter wavelength resonant above 890 MHz.

11. A tunable resonant circuit as defined in claim 10 wherein said thirdconductive section is selected to be onehalf wavelength resonant above931 MHz. and said fourth conductive section is selected to beone-quarter wavelength resonant above 931 MHz.

12. A tunable resonant circuit comprising:

a dielectric plate having two major surfaces;

a first and a second conductive section disposed on one of said majorsurfaces;

a conductive ground plane disposed on the other of said major surfacesopposite said first and said second sections;

a variable capacitance device coupled between said first and said secondsection and adapted to tune said tunable resonant circuit over a desiredband of frequencies;

said second section selected to be one-half wavelength resonant abovethe highest frequency in said desired band of frequencies, and saidfirst section selected to be onequarter wavelength resonant at afrequency above the highest frequency in said desired band offrequencies.

13. A tunable resonant circuit as defined in claim 12 wherein saidcircuit resonates with a voltage null on said second conductive sectionabove a predetermined frequency and said first conductive section iselectrically connected to said conductive ground plane.

14. A tunable resonant circuit as defined in claim 13 wherein saidvoltage null occurs on said second conductive section at a point locatedbetween the center and variable capacitance device end of said section.

15. A tunable resonant circuit as defined in claim 14 wherein said firstand said second conductive sections are axially aligned.

16. A tunable resonant circuit as defined in claim 15 wherein saidpredetermined frequency is approximately the center frequency of saiddesired band of frequencies.

17. A television tuner of the type wherein received television signalsare processed in a preselector stage to be thereafter heterodyned in amixer stage with a locally generated signal from an oscillator stage toproduce an intermediate frequency signal, said tuner including a doubletuned tunable resonant circuit coupled between said preselector andmixer stages, comprising:

a dielectric plate having two major surfaces;

a first, a second, a thirdand a fourth conductive section disposed onone of said major surfaces;

a conductive ground plane disposed on the other of said major surfacesopposite said conductive sections;

a first variable capacitance device coupled between said first and saidsecond section, and a second variable capacitance device coupled betweensaid third and said fourth section; and

a first coupling means interconnecting said first and said thirdconductive sections, and a second coupling means interconnecting saidsecond and said fourth conductive sections.

18. A double tuned tunable resonant circuit as defined in claim 17wherein said first and third sections are electrically connected andincluding means providing a conductive path between said first and thirdsections and said conductive ground plane.

19. A double tuned tunable resonant circuit adapted to be tuned across adesired band offrequencies, comprising:

a dielectric plate having two major surfaces with a conductive groundplane disposed on one of the surfaces;

a first tunable resonant circuit including a first and a secondconductive section disposed on the other surface coupled by a firstvariable capacitance device, and a second tunable resonant circuitincluding a third and a fourth conductive section disposed on the othersurface coupled by a second variable capacitance device; and

a first coupling means interconnecting the first and third conductivesections to provide dominant coupling toward the upper end of saiddesired band of frequencies, and a second coupling means interconnectingthe second and fourth conductive sections to provide dominant couplingtoward the lower end of said desired band of frequencies, said first andsaid second coupling means cooperating to provide substantially uniformcoupling between said first and said second tunable resonant circuitsacross said desired band of frequencies.

20. A double tuned tunable resonant circuit as defined in claim 19wherein said first coupling means includes an inductor.

21. A double tuned tunable resonant circuit as defined in claim 20wherein said second coupling means includes an inductor.

22. In a system including at least two tuned circuits wherein theinteraction between the tuned circuits must be controlled, a structurecomprising:

a dielectric plate having a first and a second surface;

a first tuned circuit including at least one conductive section disposedon said first face of said dielectric plate and a first conductiveground plane disposed on said second face of said dielectric plateopposite said one conductive section on said first face of saiddielectric plate; and

a second tuned circuit including at least one conductive sectiondisposed on said second face of said dielectric plate and a secondconductive ground plane disposed on the first face of said dielectricplate opposite said one conductive section on said second face of saiddielectric plate.

23. A structure as defined in claim 22 wherein said first tuned circuitincludes another conductive section disposed on said first face of saiddielectric plate and coupled to said one section section by a variablecapacitance device; and said second tuned circuit includes anotherconductive section disposed on said second face of said dielectric plateand coupled to said one section by a variable capacitance device.

24. A structure as defined in claim 23 wherein said first tuned circuitone conductive section is disposed on said first face of said dielectricplate toward one edge of said plate and electrically connected aroundthe one edge to the oppositely disposed ground plane, and said secondtuned circuit one conductive section is disposed on said second face ofsaid dielectric plate toward another edge of said plate and electricallyconnected around the another edge to the oppositely disposed groundplane.

25. A circuit comprising:

a dielectric plate having a first and a second face;

an active device having a first and a second electrode;

an input resonant circuit including a conductive section disposed onsaid first face of said dielectric plate and a first ground planedisposed on said second face of said dielectric plate opposite saidconductive section;

first means coupling said active device first electrode to said inputresonant circuit;

an output resonant circuit including a conductive section disposed onsaid secondface of said dielectric plate and a second ground planedisposed on the first face of said dielectric plate opposite saidconductive section; and second means coupling said active device secondelectrode to said output resonant circuit.

26. A circuit as defined in claim 25 wherein said active device is abipolar transistor whose emitter electrode corresponds to said devicefirst electrode and whose collector electrode corresponds to said devicesecond electrode;

27. A circuit as defined in claim 26 wherein said input resonant circuitand output resonant circuit are tunable across a band of frequenciesranging from 470 MHz. to 890 MHz.

28. A circuit comprising:

a dielectric plate having a first and a second face;

an active device having a first and a second electrode;

an input tunable resonant circuit including a first and a secondconductive section disposed on said first face of said dielectric plateand coupled by a first variable capacitance device and a firstconductive ground plane disposed on said second face of said dielectricplate opposite said first and second conductive sections;

first means coupling said active device first electrode to said inputtunable resonant circuit;

an output tunable resonant circuit including a first and a secondconductive section disposed on said second face of said dielectric plateand coupled by a second variable capacitance device and a secondconductive ground plane disposed on said first face of said dielectricplate opposite said conductive sections; and

second means coupling said active device second electrode to said outputtunable resonant circuit.

29. A circuit as defined in claim 28 wherein said first means couplessaid active device first electrode to the input tunable resonant circuitfirst conductive section and said second means couples said activedevice second electrode to the output tunable resonant circuit secondconductive section.

30. A circuit as defined in claim 29 wherein said input tunable resonantcircuit first conductive section is disposed on said first face of saiddielectric plate toward one edge of said plate, and said output tunableresonant circuit first conductive section is disposed on said secondface of said dielectric plate toward another edge of said plate.

31. A circuit as defined in claim 30 wherein one end of said inputtunable resonant circuit first conductive section is electricallyconnected around said one edge to said oppositely disposed ground plane,and one end of said output tunable resonant circuit first conductivesection is electrically connected around said another edge to saidoppositely disposed ground plane.

32. A circuit as defined in claim 31 wherein said input tunable resonantcircuit and said output tunable resonant circuit are each tunable over aband of frequencies ranging from 470 MHZ. to 890 MHZ.

1. A tunable resonant circuit comprising: a dielectric plate; anelectrically conductive material disposed on one surface of said plateto define an area of reference potential; electrically conductivematerial disposed on the opposite surface of said plate and overlyingsaid area of reference potential defining first and second spacedconductive transmission line sections; a voltage responsive capacitancedevice coupled between said first and second sections and adapted totune said tunable resonant circuit over a desired band of frequencies;said second section selected to be one-half wavelength resonant abovethe highest frequency in said desired band of frequencies, and saidfirst section selected to be one-quarter wavelength resonant at afrequency above the highest frequency in said desired band offrequencies; means connecting said first transmission line section tosaid area of reference potential; and means including said transmissionline sections for applying a voltage to said voltage responsivecapacitance device to control the resonant frequency of said tunableresonant circuit.
 2. A tunable resonant circuit as defined in claim 1wherein said pair of spaced transmission line sections are elongatedaxially aligned sections extending from one edge of said plate towardthe opposite edge thereof, said first transmission line sectionconnected to said area of reference potential by a conductive patharound said one edge of said plate.
 3. A tunable resonant circuit asdefined in claim 1 wherein said second transmission line conductivesection is dimensioned to be one-half wavelength resonant above 890 MHz.and said first transmission line conductive section is selected to beone-quarter wavelength resonant above 890 MHz.
 4. A tunable resonantcircuit as defined in claim 1 wherein said second transmission lineconductive section is dimensioned to be one-half wavelength resonantabove 931 MHz. and said first transmission line conductive section isselected to be one-quarter wavelength resonant above 931 MHz.
 5. A tunercomprising: a dielectric plate having a first and a second face; a firstand a second conductive section disposed on said first face of saiddielectric plate; a first conductive ground plane disposed on saidsecond face of said dielectric plate opposite said first and said secondconductive sections; a first variable capacitance device coupled betweensaid first and said second sections such that said first and said secondsections and said first ground plane cooperate to form a first tunableresonant circuit; a third and a fourth conductive section disposed onsaid second face of said dielectric plate; a second conductive groundplane disposed on said first face of said dielectric plate opposite saidthird and fourth sections; and a second variable capacitance devicecoupled between said third and fourth sections such that said third andsaid fourth sections and said second ground plane cooperate to form asecond tunable resonant circuit.
 6. A tunable resonant circuit asdefined in claim 5 wherein said first and said second conductivesections are axially aligned and said third and said fourth conductivesections are axially aligned.
 7. A tunable resonant circuit as definedin claim 6 wherein one conductive section of each tunable resonantcircuit resonates with a voltage null on said section above apredetermined frequency and the other conductive section is electricallyconnected to the oppositely disposed conductive ground plane.
 8. Atunable resonant circuit as defined in claim 7 wherein said voltage nulloccurs on said one conductive section at a point located between thecenter and variable capacitance device end of said section.
 9. A tunableresonant circuit as defined in claim 8 wherein said predeterminedfrequency is approximately the center frequency of said desired band offrequencies.
 10. A tunable resonant circuit as defined in claim 9wherein said first conductive section is selected to be one-halfwavelength resonant above 890 MHz. and said second conductive section isselected to be one-quarter wavelength resonant above 890 MHz.
 11. Atunable resonant circuit as defined in claim 10 wherein said thirdconductive section is selected to be one-half wavelength resonant above931 MHz. and said fourth conductive section is selected to beone-quarter wavelength resonant above 931 MHz.
 12. A tunable resonantcircuit comprising: a dielectric plate having two major surfaces; afirst and a second conductive section disposed on one of said majorsurfaces; a conductive ground plane disposed on the other of said majorsurfaces opposite said first and said second sections; a variablecapacitance device coupled between said first and said second sectionand adapted to tune said tunable resonant circuit over a desired band offrequencies; said second section selected to be one-half wavelengthresonant above the highest frequency in said desired band offrequencies, and said first section selected to be one-quarterwavelength resonant at a frequency above the highest frequency in saiddesired band of frequencies.
 13. A tunable resonant circuit as definedin claim 12 wherein said circuit resonates with a voltage null on saidsecond conductive section above a predetermined frequency and said firstconductive section is electrically connected to said conductive groundplane.
 14. A tunable resonant circuit as defined in claim 13 whereinsaid voltage null occurs on said second conductive section at a pointlocated between the center and variable capacitance device end of saidsection.
 15. A tunable resonant circuit as defined in claim 14 whereinsaid first and said second conductive sections are axially aligned. 16.A tunable resonant circuit as defined in claim 15 wherein saidpredetermined frequency is approximately the center frequency of saiddesired band of frequencies.
 17. A television tuner of the type whereinreceived television signals are processed in a preselector stage to bethereafter heterodyned in a mixer stage with a locally generated signalfrom an oscillator stage to produce an intermediate frequency signal,said tuner including a double tuned tunable resonant circuit coupledbetween said preselector and mixer stages, comprising: a dielectricplate having two major surfaces; a first, a second, a third and a fourthconduCtive section disposed on one of said major surfaces; a conductiveground plane disposed on the other of said major surfaces opposite saidconductive sections; a first variable capacitance device coupled betweensaid first and said second section, and a second variable capacitancedevice coupled between said third and said fourth section; and a firstcoupling means interconnecting said first and said third conductivesections, and a second coupling means interconnecting said second andsaid fourth conductive sections.
 18. A double tuned tunable resonantcircuit as defined in claim 17 wherein said first and third sections areelectrically connected and including means providing a conductive pathbetween said first and third sections and said conductive ground plane.19. A double tuned tunable resonant circuit adapted to be tuned across adesired band of frequencies, comprising: a dielectric plate having twomajor surfaces with a conductive ground plane disposed on one of thesurfaces; a first tunable resonant circuit including a first and asecond conductive section disposed on the other surface coupled by afirst variable capacitance device, and a second tunable resonant circuitincluding a third and a fourth conductive section disposed on the othersurface coupled by a second variable capacitance device; and a firstcoupling means interconnecting the first and third conductive sectionsto provide dominant coupling toward the upper end of said desired bandof frequencies, and a second coupling means interconnecting the secondand fourth conductive sections to provide dominant coupling toward thelower end of said desired band of frequencies, said first and saidsecond coupling means cooperating to provide substantially uniformcoupling between said first and said second tunable resonant circuitsacross said desired band of frequencies.
 20. A double tuned tunableresonant circuit as defined in claim 19 wherein said first couplingmeans includes an inductor.
 21. A double tuned tunable resonant circuitas defined in claim 20 wherein said second coupling means includes aninductor.
 22. In a system including at least two tuned circuits whereinthe interaction between the tuned circuits must be controlled, astructure comprising: a dielectric plate having a first and a secondsurface; a first tuned circuit including at least one conductive sectiondisposed on said first face of said dielectric plate and a firstconductive ground plane disposed on said second face of said dielectricplate opposite said one conductive section on said first face of saiddielectric plate; and a second tuned circuit including at least oneconductive section disposed on said second face of said dielectric plateand a second conductive ground plane disposed on the first face of saiddielectric plate opposite said one conductive section on said secondface of said dielectric plate.
 23. A structure as defined in claim 22wherein said first tuned circuit includes another conductive sectiondisposed on said first face of said dielectric plate and coupled to saidone section section by a variable capacitance device; and said secondtuned circuit includes another conductive section disposed on saidsecond face of said dielectric plate and coupled to said one section bya variable capacitance device.
 24. A structure as defined in claim 23wherein said first tuned circuit one conductive section is disposed onsaid first face of said dielectric plate toward one edge of said plateand electrically connected around the one edge to the oppositelydisposed ground plane, and said second tuned circuit one conductivesection is disposed on said second face of said dielectric plate towardanother edge of said plate and electrically connected around the anotheredge to the oppositely disposed ground plane.
 25. A circuit comprising:a dielectric plate having a first and a second face; an active devicehaving a first and a second elEctrode; an input resonant circuitincluding a conductive section disposed on said first face of saiddielectric plate and a first ground plane disposed on said second faceof said dielectric plate opposite said conductive section; first meanscoupling said active device first electrode to said input resonantcircuit; an output resonant circuit including a conductive sectiondisposed on said second face of said dielectric plate and a secondground plane disposed on the first face of said dielectric plateopposite said conductive section; and second means coupling said activedevice second electrode to said output resonant circuit.
 26. A circuitas defined in claim 25 wherein said active device is a bipolartransistor whose emitter electrode corresponds to said device firstelectrode and whose collector electrode corresponds to said devicesecond electrode;
 27. A circuit as defined in claim 26 wherein saidinput resonant circuit and output resonant circuit are tunable across aband of frequencies ranging from 470 MHz. to 890 MHz.
 28. A circuitcomprising: a dielectric plate having a first and a second face; anactive device having a first and a second electrode; an input tunableresonant circuit including a first and a second conductive sectiondisposed on said first face of said dielectric plate and coupled by afirst variable capacitance device and a first conductive ground planedisposed on said second face of said dielectric plate opposite saidfirst and second conductive sections; first means coupling said activedevice first electrode to said input tunable resonant circuit; an outputtunable resonant circuit including a first and a second conductivesection disposed on said second face of said dielectric plate andcoupled by a second variable capacitance device and a second conductiveground plane disposed on said first face of said dielectric plateopposite said conductive sections; and second means coupling said activedevice second electrode to said output tunable resonant circuit.
 29. Acircuit as defined in claim 28 wherein said first means couples saidactive device first electrode to the input tunable resonant circuitfirst conductive section and said second means couples said activedevice second electrode to the output tunable resonant circuit secondconductive section.
 30. A circuit as defined in claim 29 wherein saidinput tunable resonant circuit first conductive section is disposed onsaid first face of said dielectric plate toward one edge of said plate,and said output tunable resonant circuit first conductive section isdisposed on said second face of said dielectric plate toward anotheredge of said plate.
 31. A circuit as defined in claim 30 wherein one endof said input tunable resonant circuit first conductive section iselectrically connected around said one edge to said oppositely disposedground plane, and one end of said output tunable resonant circuit firstconductive section is electrically connected around said another edge tosaid oppositely disposed ground plane.
 32. A circuit as defined in claim31 wherein said input tunable resonant circuit and said output tunableresonant circuit are each tunable over a band of frequencies rangingfrom 470 MHz. to 890 MHz.